Heart failure is one of the highest causes of death worldwide. A common form of heart failure occurs due to the malfunctioning of diseased heart valves, and it is estimated that between 275,000 and 370,000 valve replacement procedures are performed each year. During such procedures, a prosthetic valve is implanted into an annular opening in the heart of a patient, typically after surgical removal of a diseased, damaged or malfunctioning natural valve.
Traditional heart valve replacement methods, which are still widely used today, require that a patient undergo open heart surgery. Open heart surgery involves the physical separation of a patient's breastbone so as to allow access to the heart. This replacement method not only leaves considerable scars, but may also pose a risk to infection, bleeding as well as requiring long hospitalisation and recovery periods.
In order to overcome the disadvantages and risks of open heart surgery, a number of minimally invasive techniques or procedures have been developed. The most common forms of minimally invasive procedures used for heart valve replacement are known as endovascular procedures, where the incision or access point is through a blood vessel, such as the femoral artery. The procedure is carried out percutaneously and transluminally using the vascular system to convey appropriate devices to the desired position in the body. Minimally invasive surgical techniques include transapical techniques, whereby a mini-thoracotomy is performed and the replacement heart valve is inserted directly through the apex of the heart.
The use of endovascular procedures during heart valve replacement has led to the development of replacement valves capable of being reduced in size, also referred to as crimping, so as to be capable of being moved through a blood vessel and then later to be expanded to a desired size once deployed. Expandable heart valves currently used typically make use of self-expandable materials for the valve stent and are compressed into a valve deployment device. Balloon-expandable valves, for which materials such as stainless steel or nickel-cobalt-chromium alloys are used for the stent, are also commonly used.
Leaflets, typically made from stabilized tissue of animal origin, such as bovine or porcine pericardium, are attached to the stent typically by means of stitching the leaflets onto the stent frame, normally along a pericardium or fabric skirt, or by means of direct stitching to the stent frame in the case of xenograft valves. Polymeric leaflets have also been proposed which may be attached to the stent by means of stitching, gluing or other means.
In the case of tri-leaflet valves, the leaflets are generally attached to three shaped posts of the stem along an attachment edge and adjacent leaflets typically join at such posts to provide a commissure. The leaflet edge extending between the two commissures is generally referred to as the free edge, while the generally curved leaflet area between the free edge and the attachment edge is known as the belly.
Various leaflet designs, particularly polymeric leaflets, have been developed in recent years, however, these have a number of potential drawbacks. Designs causing overly close coaptation of the leaflets may limit wash-out of blood during haemodynamic function, particularly in the regions close to the stent posts at the commissures. These regions, also referred to as regions of stagnation, may encourage local thrombogenesis, and may lead to further restriction of the valve orifice in the longer term.
Another disadvantage of certain existing designs is that the valve leaflets may not fully close in the coaptive region. This may result in excessive regurgitation upon closure of the artificial valve. Furthermore, valve leaflet design may cause high stresses at commissures or in regions of the leaflet belly, which may lead to leaflet damage or valve malfunction.
Some leaflet designs may lead to insufficient orifice size when the artificial valve leaflets are in an open position. This may result in a high pressure drop across the valve, which can in turn limit haemodynamic performance of the artificial valve.
In addition to the potential leaflet design drawbacks set out above, a major drawback experienced with minimally invasive procedures and thus prosthetic heart valves that may be introduced by such procedures, is that the procedures require highly specialised equipment which has the effect that such procedures are highly expensive. The cost of replacing a defective heart valve typically ranges from about US$60,000 to US$100,000, thus making such procedures generally only available to the developed world.
The term “polymer” in this specification shall have its widest meaning and includes plastics materials suitable for use in the human body, such as polyurethanes, and also includes reinforced polymers, such as fibre reinforced polymers, and composites constructed using polymers.